German Patent Application No. DE 198 34 276, for example, describes a sensor element. The planar, elongated sensor element contains an electrochemical cell having a first and a second electrode, as well as a solid electrolyte located between the first and the second electrode. Furthermore, the sensor element contains a wave-form heater situated between two insulation layers.
The heater and the layers adjacent to the heater are exposed during operation to high mechanical stresses, which result from temperature fluctuations and the temperature gradients occurring within the layers. To prevent cracks, for example, from occurring in the insulation layers due to these stresses, the insulation layers have a porous design and therefore have adequate elasticity.
The heater is connected by two heater leads to an electric circuit situated outside of the sensor element. The sensor element is heated to a predefined temperature by applying a predefined heating voltage to the heater in a switched mode. The heating voltage is regulated by determining the temperature-dependent internal resistance of the electrochemical cell.
The insulation layers are made of aluminum oxide, for example, and contain contaminants such as sodium due to the manufacturing process. When a heating voltage is applied, the voltage drop in the heater area may cause mobile ions such as sodium ions to wander within the heating insulation and accumulate on one side of the heater, for example. Due to the polarization of the heater insulation, when the heater is operated in a switched mode (sudden switching on and off of the heater voltage), the heater may inject capacitive interference into the electrochemical cell.
Polarization in the area of the heater insulation also occurs during leak current testing, when a high voltage is applied between the heater and one electrode of the electrochemical cell.